Spatial and temporal distribution of fish and zooplankton in a shallow lake PIA ROMARE*, SØREN BERG † , TORBEN LAURIDSEN ‡ AND ERIK JEPPESEN ‡, § *Department of Biology, University of Washington, Seattle, WA, U.S.A. †Department of Inland Fisheries, Danish Institute for Fisheries Research, Vejlsøvej, Silkeborg, Denmark ‡Department of Freshwater Ecology, National Environmental Research Institute, Vejlsøvej, Silkeborg, Denmark §Department of Botanical Ecology, University of Aarhus, Nordlandsvej, Risskov, Denmark SUMMARY 1. We performed both a large- and a small-scale echo sounding study on the spatial and temporal distribution of fish (mainly roach, Rutilus rutilus and perch, Perca fluviatlis), as well as a small-scale study of zooplankton distribution in the small, shallow and eutrophic Lake Hanebjerg in Denmark. In the small-scale study, sampling was conducted in open water as well as in the edge zone immediately outside two different types of vegetation. 2. Fish daytime abundances differed between the northern and the southern parts of the lake and, on a small scale, small fish aggregated in the edge zones during day, preferably outside dense emergent vegetation. Copepods avoided emergent vegetation, while cladocerans showed no habitat preference. Both small fish and cladoceran numbers were found to be higher during night than day. 3. The relative abundance (number per sample) of cladocerans in the edge zone immediately outside vegetation was negatively correlated with the relative abundance of fish in that zone. There was no correlation between cladoceran and fish abundance in open water, or between the relative abundance of copepods and fish. 4. The presence of pelagic piscivores in combination with avoidance behaviour of both fish and zooplankton is a likely explanation for the observed distribution of small fish and cladocerans in Lake Hanebjerg. Both small- and large-scale distribution patterns may be dependent on the type and distribution of complex structure in the lake. Even in a small lake, large-scale patterns may affect the interpretation of small-scale data. Keywords: cladocerans, distribution, littoral–pelagic coupling, littoral landscape, roach Introduction The spatial distribution and abundance of organisms in ecosystems are of crucial importance for under- standing ecosystem functioning (Rosenzweig, 1991). In small lakes the distance between the pelagic and the littoral habitat is short and in shallow lakes abiotic factors do not shape horizontal gradients as strongly as do the vertical gradients in deep lakes (Burks et al., 2002). Thus, in small, shallow lakes biotic factors are more important than was thought earlier (see review in Burks et al., 2002), and the littoral zone and its coupling with the pelagic habitat (horizontal migra- tion, ontogenetic habitat shifts) become important for shaping the distribution patterns of fish and zooplankton (review in Jeppesen et al., 1997; Masson et al., 2001). To identify the factors influencing the distribution pattern of lake organisms it is important to view the littoral zone as a mosaic of habitats, ‘a littoral landscape’, (Lodge et al., 1988; Chick & McIvor, 1994) with different types of structural complexity. It is well-known that macrophytes may act as a refuge from predation for both small fish and zooplankton Correspondence: Pia Romare, Department of Biology, Univer- sity of Washington, Box 351800, Seattle, WA 98195-1800, U.S.A. E-mail: piaromar@u.washington.edu Freshwater Biology (2003) 48, 1353–1362 Ó 2003 Blackwell Publishing Ltd 1353